function [ a, b, c, d, t0, y0, tstop ] = fitzhugh_nagumo_parameters ( ...
  a_user, b_user, c_user, d_user, t0_user, y0_user, tstop_user )


%*****************************************************************************80
%
%% fitzhugh_nagumo_parameters() returns parameters of the Fitzhugh-Nagumo ODE.
%
%  Discussion:
%
%    If input values are specified, this resets the default parameters.
%    Otherwise, the output will be the current defaults.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    23 April 2021
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    real A_USER, B_USER, C_USER, D_USER: the parameters.
%
%    real T0_USER: the initial time.
%
%    real Y0_USER(2): the initial condition.
%
%    real TSTOP_USER: the final time.
%
%  Output:
%
%    real A, B, C, D: the parameters.
%
%    real T0: the initial time.
%
%    real Y0(2): the initial condition.
%
%    real TSTOP: the final time.
%
  persistent a_default;
  persistent b_default;
  persistent c_default;
  persistent d_default;
  persistent t0_default;
  persistent y0_default;
  persistent tstop_default;
%
%  Initialize defaults.
%
  if ( isempty ( a_default ) )
    a_default = 0.7;
  end

  if ( isempty ( b_default ) )
    b_default = 0.8;
  end

  if ( isempty ( c_default ) )
    c_default = 12.5;
  end

  if ( isempty ( d_default ) )
    d_default = 0.5;
  end

  if ( isempty ( t0_default ) )
    t0_default = 0.0;
  end

  if ( isempty ( y0_default ) )
    y0_default = [ 0.0, 0.0 ];
  end

  if ( isempty ( tstop_default ) )
    tstop_default = 100.0;
  end
%
%  Update defaults if input was supplied.
%
  if ( 1 <= nargin )
    a_default = a_user;
  end

  if ( 2 <= nargin )
    b_default = b_user;
  end

  if ( 3 <= nargin )
    c_default = c_user;
  end

  if ( 4 <= nargin )
    d_default = d_user;
  end

  if ( 5 <= nargin )
    t0_default = t0_user;
  end

  if ( 6 <= nargin )
    y0_default = y0_user;
  end

  if ( 7 <= nargin )
    tstop_default = tstop_user;
  end
%
%  Return values.
%
  a = a_default;
  b = b_default;
  c = c_default;
  d = d_default;
  t0 = t0_default;
  y0 = y0_default;
  tstop = tstop_default;
  
  return
end

